1. Field of the Invention
This invention is in the field of anti-backlash nuts designed to provide uniform translational movement in either direction along a threaded screw in response to relative rotational movement between the nut and screw.
2. Description of the Prior Art
In many applications, it is important to drive an element of a machine along a screw with accurate positional repeatability and constant drag torque in the forward and reverse direction. Data printers and x-y tables, used as peripheral equipment in the computer industry, for example, have such requirements.
Positioning devices designed to meet these requirements have been proposed, and many of these employ an anti-backlash nut to achieve the positional accuracy along the screw which is required. Examples of two such anti-backlash nut assemblies which have been proposed are described in the patent literature as follows.
In U.S. Pat. No. 3,656,358, issued to Kopp, a linear positioning device is disclosed which is stated to have an improved collar for use with a comparatively inexpensive rod having multiple grooves. The collar is telescoped over and adapted to be translated back and forth relative to the elongated rod. This collar includes cantilevered fingers which are resiliently wedged into angularly spaced grooves formed in the rod to preload the collar onto the rod and prevent rotational play from developing between the two. In a specific embodiment, the collar is telescoped onto a rod in the form of a splined shaft while in another embodiment, the collar is a nut threaded onto a screw with multiple threads.
In U.S. Pat. No. 3,997,269, issued to Linley, an anti-backlash, self-aligning nut construction with specially constructed tubular nut bodies which coact with concentric spring sleeves is described. The nut bodies, in general, each have a pair of spring-biased elements provided with internal thread formations adapted for engagement with the external threads of a screw. In one embodiment, a self-aligning spring sleeve is provided having solely three pairs of oppositely-disposed transverse slots to obtain the desired aligning features. The nut body has a base portion which is separated from the spring-biased elements by means of two transverse slots which, together with an adjacent pair of slots in the spring sleeve, form in effect a universal joint. One of the remaining slot pairs in the sleeve is oriented circumferentially with respect to the first pair by an angle of 90.degree., with a third pair of slots being circumferentially aligned with the first pair.
More recently, an anti-backlash nut having oppositely-directed longitudinal flexure members has been disclosed in U.S. application Ser. No. 907,518, filed May 19, 1978. This anti-backlash nut has a continuous portion extending longitudinally from one end of the screw to the other. In addition, there are at least two, and usually more, oppositely-directed longitudinal flexure members which have one end fixed to the anti-backlash nut and one end free-floating. The oppositely-directed longitudinal flexure members are biased towards the screw by one or more radial springs or other means for biasing.
While the anti-backlash nut described in U.S. application Ser. No. 907,518 has proven to be very successful, it sometimes suffers a problem after the threads on the nut have become worn. This problem relates to the fact that when an axial load is applied to this nut there is an opposite force of reaction directed towards the inclined surface of the nut thread. Since this surface of the nut thread is inclined, the force of reaction is broken into an axial or longitudinal force vector and a normal force vector. The normal force vector tends to force the nut away from the screw, and in many cases can overcome the normal force towards the screw which is created by the radial spring.